Bellows, serving especially for, the protection of parts of machine tools, are provided with supporting frames for stiffening and achieving a good inherent stability which are inserted from the inside into the outside facing pleats. The form of these supporting frames matches the profile of the track against which the supporting frames rest during operation. Bellows, with a bottom-side open accordion which consists of a cover and two lateral walls, have generally U-shaped supporting frames. In the corner areas of the bellows --due to the folding of the accordion- several layers of material of the accordion lie superimposed. Additionally, there is the material thickness of the supporting frames. The compression size of the bellows, this means the minimum axial length to which the bellows are compressible, is, therefore, limited, because of the numbers of the layers of material in the corner areas of the bellows.